A wireless receiver includes an antenna system that receives a signal of interest as well as background noise and interference. The wireless receiver therefore includes a tuner having one or more BPFs to isolate the signal of interest.
When designing a BPF, ideally the bandwidth (BW) of the pass-band should be equal to the BW of the signal of interest. One challenge to achieving this goal is tuning the BPF so that the pass-band is centered at the carrier frequency (f0) of the signal of interest. If the pass-band is not centered, then the BPF introduces distortion to the signal of interest.
The challenge of centering the pass-band can be mitigated by increasing the BW of the BPF so that it is wider than the bandwidth of the signal of interest. However, increasing the BW of the BPF also increases the portion of undesirable signals that pass through it. These undesirable signals then reduce the linearity of the receiver. This distortion decreases the signal-to-noise ratio of the receiver and is at odds with the desire to maximize linearity of the receiver mode.
Referring now to FIG. 1, a radio-frequency (RF) tuner 10 of a receiver is shown. A signal Rx is received by an antenna system (not shown) and includes the signal of interest and the undesirable signals. The signal Rx is applied to an input of a first BPF 12 that rejects a substantial portion of the undesirable signals. An output of the first BPF 12 communicates with a low noise amplifier (LNA) 14. The LNA 14 amplifies the signal and applies it to a second BPF 16. The second BPF 16 generally has a narrower pass-band than the first BPF 12. The second BPF 16 communicates the signal of interest to a mixer 18. The mixer 18 receives a fixed-frequency signal from a local oscillator 20 and downconverts the frequency of the signal of interest. The mixer then provides the downconverted signal of interest to a baseband receiver 22 to be demodulated.
The linearity of the RF tuner 10 is dependent on the pass-bands of the first and second BPFs 12, 16 being centered upon the signal of interest and having the smallest possible BWs.
Referring now to FIG. 2, a schematic diagram is shown of a simple BPF 30. The received signal Rx is applied across a first parallel combination that includes a first inductor 32 and first adjustable capacitor 34. One end of the first parallel combinations connected to a reference node, such as a ground 40. The opposite end of the first parallel combination is connected to one end of a series combination that includes a second inductor 44 and a second adjustable capacitor 46. The opposite end of the series combination is connected to one end of a second parallel combination that includes a second inductor 50 and a second adjustable capacitor 52. The opposite end of the second parallel combination is connected to ground 40. Each of the adjustable capacitors 34, 46, and 52 can be implemented with a varactor and an associated bias voltage.
The components of the BPF 30 are selected and adjusted to simultaneously pass the signal of interest to output terminals 54 and shunt as much of the undesirable signals to ground 40. The effectiveness of the BPF 30 is affected by the accuracy of the capacitances of the first, second, and third adjustable capacitors 34, 46, and 52, respectively. If their capacitances vary, such as may occur over time, temperature, and/or humidity, the center frequency (fc) of the BPF 30 will shift and its performance will deteriorate.